In the transmission of radio signals such as in FM broadcasting, there is a requirement for an antenna which is omnidirectional in pattern and which presents a low VSWR to the feed line and transmitter over a broad band of frequencies.
It is well known that a combination of vertical and horizontal polarization (with respect to some reference plane, e.g., the earth's surface) will result in circular or elliptical polarization of radiation, depending upon the relative phase and amplitude relationship. Vertically polarized radiation may be obtained from a vertical antenna such as a vertically oriented dipole; horizontally polarized radiation may be obtained from a horizontal antenna such as a horizontally oriented dipole or loop. The loop may be bent into a complete circle, or be peripherally incomplete with the open ends of the semicircular conductors being connected together electrically by capacitance plates. Either loop form has a toroidal radiation pattern polarized in the plane of the omnidirectional pattern and of the loop. A vertical dipole has the same radiation pattern but polarized transverse to the plane of the omnidirectional pattern.
It has been suggested in the literature that these two antenna types with identical radiation patterns when orthogonally polarized be combined to form a circularly polarized antenna, omnidirectional in one plane, usually the horizontal plane. To do this, it is necessary to so locate the loop and dipole that their phase centers are essentially coincident and then to feed the two with equal signal, and 90.degree. phase difference. This arrangement is difficult to achieve.
In the past FM transmitting antennas having a VSWR of 1.5:1 over a bandwidth of .+-.1% of the frequency of interest has been regarded as reasonable. Thus, for the FM band (88-108 mc) the VSWR would be under 1.5:1 for a band of about 2 mc.
In Richard D. Bogner U.S. Pat. No. 3,474,452 issued Oct. 21, 1969, and assigned to the assignee of the present application, it has been disclosed that it is possible to obtain an omnidirectional circularly polarized antenna from a combination of a horizontally polarized loop with vertically polarized elements affixed to each end of the loop. Similarly, a loop can be bent into helical section which will produce a circularly polarized omnidirectional pattern. However, such a structure generally results in a VSWR in the order of 1.5:1 over the aforedefined "resonable" bandwidth. If the loop is opened up into an arc of much larger diameter, such as about 30 inches diameter at 100 mHz, for example, the bandwidth will be increased by an order of about 5:1. However, an unacceptable radiation pattern results.
It is desirable, however, that a broad bandwidth be achievable with low VSWR. If the bandwidth is in the order of 10 mHz in the FM band, a single antenna may be employed for the simultaneous radiation of several channels. This is accomplished by feeding the output of several transmitters through suitable filters to a single antenna. Further, a broad band antenna is less susceptable to the deleterous effects of ice formation or the effects of tower proximity. Another advantage is that only a few designs are necessary to provide antennas covering the entire FM band of 88 to 108 mHz, thus simplifying the manufacturing process.
The antenna of this invention employs a pair of helical dipole loops positioned at an angle relative to each other and fed 180.degree. out of phase with each other. The antenna exhibits broader bandwidth and a radiation pattern of better circularity than that obtainable with a single loop antenna. By forming the loop of tubing of large cross-sectional diameter as for example of 3 1/8 inches tubing as employed for coaxial transmission line, the broad bandwidth characteristic is maintained.
A feature of the invention is that the antenna is fed entirely from circuitry contained within the antenna element itself, leaving exposed to the elements only two insulated sleeves; or, in an alternative embodiment, through the use of a balun, four insulated sleeves. Accordingly, it is an object of this invention to provide an antenna which has broad bandwidth with low VSWR and which has a radiation pattern of better circularity than that obtainable with a single loop.
An additional feature of the invention is that an approximately one-fourth wave long, horizontal supporting section is available in which a matching section may be placed to transform the single element feed impedance up from the single bay self-impedance of approximately 50 ohms, to N times 50 ohms, where N is the number of elements above or below the feed point thus facilitating feeding of an array from a standard 50 ohm transmission line.
If it were desired to employ a different impedance line, say of 25 ohm characteristic impedance, then the impedance of the dipole elements and/or the transformer would be adjusted accordingly.
As will be discussed more fully hereinafter, the low operating Q of the antenna in conjunction with its braod bandwidth allow it to be operated with an unusually high input power per bay, typically 40 kilowatts per bay.
A most important further feature of this antenna is that the broad bandwidth and sleeve protected feed make it highly impervious to icing, so that it may be used in moderate icing areas without the addition of thermal deicing or the addition of protective radomes. For instance, radomes in high icing areas are often subject to severe damage from falling ice. This feature leads, in turn, to a considerable simplification in maintenance.
The extremely wide operational bandwidth of this antenna coupled with the extremely high per bay power handling capability makes it suitable for multi-station use, provided that suitable combining filters are employed.
The above description, as well as further objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred, but nonetheless illustrative, embodiment in accordance with the present invention when taken in conjunction with the accompanying drawings.